1. Field of the Invention
The present invention relates to an image processing apparatus and image processing method, which generate image data required to form an image using a plurality of color materials.
2. Description of the Related Art
In order to obtain stable tone expression in color printing, the following technique is known. That is, positions where a color material covers paper (color material covering positions will be referred to as dots hereinafter) are arranged in a grid pattern, and respective colors are set to have different grid tilt angles. If the grid patterns of the respective colors are set to have an equal tilt angle, and dots are superposed, the positional relationship between dots becomes constant, and color misregistration directly leads to a color variation, resulting in unstable colors. That is, by changing the tilt angles of the grid patterns for respective colors to obtain different positional relationships between dots of the different colors, a variation of a degree of overlapping of dots due to slight color misregistration is canceled (for example, see Japanese Patent Laid-Open No. 6-130656).
However, there is a demerit due to the different tilt angles of the grid patterns. By tilting the grid patterns, dots have different cycles in the horizontal and vertical directions, and a characteristic pattern (color moiré pattern) having a cycle until the positional relationships among dots of the respective colors return to the same positional relationship is often visually observed.
In the press, a color material is applied to a plate having a physical bumps and dips or a plate to the flat surface of which a hydrophobic or hydrophilic property is given, and an image is transferred to a paper sheet by bringing the plate and sheet into tight contact with each other. On a printed product obtained in the press, a solvent of the color material is vaporized, and only a pigment of the color material is left on the sheet, that is, nearly no materials other than the pigment are left. On the other hand, in electrophotographic printing, a latent image optically rendered on a photosensitive member is developed by charged toner to form a toner image, the toner image is transferred onto a paper sheet, and that toner image is fixed on the sheet by heat and pressure. That is, a resin of the toner serves as an adhesive between the sheet and a pigment included in the toner. As a result, a printed product obtained by the electrophotographic printing has a larger volume of a material attached on the printing surface by the thickness of the resin component than that of a printed product obtained in the press. Furthermore, in case of electrophotographic color printing, resin components are laminated to form multi-layers. The electrophotographic printing which laminates resin components in multi-layers suffers the following problems.
Toner is flattened out by a pressure at the time of fixing. A toner spread becomes more conspicuous with increasing thickness of toner before fixing. That is, the toner spread becomes more conspicuous on a region where a plurality of color toners overlap. As a result, such toner spread influences tone expression based on covering ratios of pigments on a small region. More specifically, since a pigment is also flattened out together with toner, the covering ratio of that pigment increases, resulting in an increase in density of an image.
An increase in covering ratio of the pigment is also caused by scattering of toner at the time of development and fixing. A region where a plurality of color toners overlap (overlapping portion) has a larger toner scattering amount than a region where only one color toner exists (non-overlapping portion). As a result, even when these two regions have the same target density, the density of the overlapping portion becomes higher than that of the non-overlapping portion.
A printing area is often larger or smaller than a rendering area. However, in the electrophotographic color printing, a spreading pattern of the printing area is not uniform, and depends on overlapping of toners. Especially, on a portion where respective color dots overlap, the printing area readily spreads, and has a higher density, resulting in a conspicuous density change due to interference between colors. As a result of such density change, a color moiré pattern appears as a stronger interference pattern. Even in a color combination that does not cause any color moiré pattern on a printed product of the press, the color moiré pattern often becomes obvious on a printed product of the electrophotographic printing.
As a technique for eliminating a color moiré pattern, that using a liquid developing agent is available (for example, Japanese Patent Laid-Open No. 2006-341521, patent reference 2). The liquid developing agent allows to form a thinner toner layer than dry toner, thus providing an effect of suppressing a density change on the overlapping portion.
Also, a technique for eliminating a color moiré pattern by suppressing the amount of applied toners on a color overlapping portion is available (for example, Japanese Patent Laid-Open No. 8-298595, patent reference 3). This technique eliminates the color moiré pattern by decreasing the area of an overlapping portion without printing a Y-color of a portion where, for example, K- and Y-colors overlap.
Furthermore, a technique that obscures the color moiré pattern by expelling the frequency of the color moiré pattern to a high-frequency region is available (for example, Japanese Patent Laid-Open No. 2002-112047, patent reference 4). With this technique, for example, screens which convert the color moiré pattern caused by three colors into high-frequency components are used as C-, M-, and K-colors, and a screen angle of a Y-color is set to be equal to one of the C-, M-, and K-colors to shift a phase, thereby expelling the frequency of the color moiré pattern to the high-frequency region.
However, it is difficult for the technique that eliminates or obscures the color moiré pattern to express a dot image equivalent to a printed product of the press. The invention of patent reference 2 requires not only the special liquid developing agent but also a process for drying the liquid developing agent and that for a vaporized solvent upon drying, resulting in high cost.
With the technique described in patent reference 3, when a pattern of the overlapping portion exists in a high-frequency region, if color misregistration occurs, a reverse effect, that is, so-called a highlight detail loss occurs due to elimination of a color moiré pattern. The invention of patent reference 3 skips correction for eliminating a color moiré pattern when color misregistration is large in consideration of occurrence of color misregistration. In other words, in order to obtain a color moiré elimination effect by the invention of patent reference 3, accurate registration is required. Furthermore, by correction processing that suppresses the area of the overlapping portion, a dot shape changes, and unstable output patterns such as isolated points are likely to be formed.
The invention of patent reference 4 suffers a problem of a large color change when misregistration of the Y-color occurs. The screens of patent reference 4 cannot express a dot image as in the press unlike those used in the press.